hi all,

just bumping in on this as I do have a deviant opinion, from the image below. It shows four standard boxes under default lighting, each box is 0.1 PNU (10.2" or 26.2cm) in size.
The colored one at the back is for illustration only, it has a simple tiled pattern assigned to its Bump as well as Diffuse channel so I can see what's dark and what's bright. The gray mortar is a bit brightened up, so it makes about 50% under Gamma Correction.

The boxes at the front are put closely to each other, the left one does not have any bump, the middle one has a very stong bump: 2,62 mtr (thats 10x the box size!) while the right one is still quite bumpy: 0,262 (thats 1x the box size). My units are meters. 

 

 

What do I see:

• both the strong bump effects show equal effects
• all bump effect gets completely lost when a box boundary is crossed. Especially from the middle to the left box there is no clue whether a surface on the middle box is raised, indended or 'neutral'.

My conclusions:

• the bump effect is a fake one, making a surface element look raised or lowered relative to a neighboring surface element, depending on the relative (!) brightnesses within the same bump map.
• the fake effect is limited. One can have it 10% fake, or 50% fake, but there is some 100% fake level which cannot be exceeded. Higher values do not make stronger bump effects. 
• the bump effect is limited to surface elements addressed by the same bump map. It does not make elements look raised or indended relative to elements addressed by another map.
• in other words: their is just no such thing as neutral. As the boundary between the left and middel box makes clear: white, gray and black are equally neutral, they have only meaning relative to each other within the same mapped area. 
• the story on distorted normals is a nice one, but not explaining why I don't see distortions on the box boundaries. So, to me at the moment, I'm not buying, and bump is just a fake shadowing / highlighting effect. Nice, but fake.

 

• on top of all this, do remind that GC, IDL as well as SSS all reduce the highlight / shadow contrasts, and hence tend to reduce the effects of bump (and displacement, and overall image depth, etc).

 That's my opinion on bump.

 

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though